If you are experiencing painful swelling in the back of your foot just above the heel, you may be suffering from Achilles Tendonitis. This tendon is the strongest and largest tendon in the body and connects to leg to the foot as it stretches from the lower leg to heel bone. This condition typically develops gradually as the tendon inflammation and may include one or more of three stages: Peritonitis - pain during or after activity, Tendinitis -swelling at the back of the leg &Peritenonitis with Tendinitis - may lead to a rupture of tendon. Achilles tendonitis is best treated early in the development of the condition.